From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Hansen <dave.hansen@linux.intel.com>
Subject: Re: [RFC PATCH v3 12/24] x86/mm: Modify ptep_set_wrprotect and
 pmdp_set_wrprotect for _PAGE_DIRTY_SW
Date: Fri, 31 Aug 2018 08:58:39 -0700
Message-ID: <6d31bd30-6d5b-bbde-1e97-1d8255eff76d@linux.intel.com>
References: <ce051b5b-feef-376f-e085-11f65a5f2215@linux.intel.com>
 <1535649960.26689.15.camel@intel.com>
 <33d45a12-513c-eba2-a2de-3d6b630e928e@linux.intel.com>
 <1535651666.27823.6.camel@intel.com>
 <CAG48ez3ixWROuQc6WZze6qPL6q0e_gCnMU4XF11JUWziePsBJg@mail.gmail.com>
 <1535660494.28258.36.camel@intel.com>
 <CAG48ez0yOuDhqxB779aO3Kss3gQ3cZTJL1VphDXQm+_M9jFPvQ@mail.gmail.com>
 <1535662366.28781.6.camel@intel.com>
 <CAG48ez0mkr95_TbLQnDGuGUd6G+eJVLZ-fEjDkwA6dSrm+9tLw@mail.gmail.com>
 <CAG48ez3S3+DzAyo_SnoUW1GO0Cpd_x0A83MOx2p_MkogoAatLQ@mail.gmail.com>
 <20180831095300.GF24124@hirez.programming.kicks-ass.net>
 <1535726032.32537.0.camel@intel.com>
 <f5a36e32-7c5f-91fe-9e98-fb44867fda11@linux.intel.com>
 <1535730524.501.13.camel@intel.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: 8bit
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <1535730524.501.13.camel@intel.com>
Content-Language: en-US
Sender: linux-kernel-owner@vger.kernel.org
To: Yu-cheng Yu <yu-cheng.yu@intel.com>, Peter Zijlstra <peterz@infradead.org>, Jann Horn <jannh@google.com>
Cc: the arch/x86 maintainers <x86@kernel.org>, "H . Peter Anvin" <hpa@zytor.com>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, kernel list <linux-kernel@vger.kernel.org>, linux-doc@vger.kernel.org, Linux-MM <linux-mm@kvack.org>, linux-arch <linux-arch@vger.kernel.org>, Linux API <linux-api@vger.kernel.org>, Arnd Bergmann <arnd@arndb.de>, Andy Lutomirski <luto@amacapital.net>, Balbir Singh <bsingharora@gmail.com>, Cyrill Gorcunov <gorcunov@gmail.com>, Florian Weimer <fweimer@redhat.com>, hjl.tools@gmail.com, Jonathan Corbet <corbet@lwn.net>, keescook@chromiun.org, Mike Kravetz <mike.kravetz@oracle.com>, Nadav Amit <nadav.amit@gmail.com>, Oleg Nesterov <oleg@redhat.com>, Pavel Machek <pavel@ucw.cz>, ravi.v.shankar@intel.com, vedvya
List-Id: linux-api@vger.kernel.org

On 08/31/2018 08:48 AM, Yu-cheng Yu wrote:
> To trigger a race in ptep_set_wrprotect(), we need to fork from one of
> three pthread siblings.
> 
> Or do we measure only how much this affects fork?
> If there is no racing, the effect should be minimal.

We don't need a race.

I think the cmpxchg will be slower, even without a race, than the code
that was there before.  The cmpxchg is a simple, straightforward
solution, but we're putting it in place of a plain memory write, which
is suboptimal.

But, before I nitpick the performance, I wanted to see if we could even
detect a delta.